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Quantifying finish and wear on oversized or inaccessible parts presents a challenge in applications from machining to shipbuilding. In some cases, measurement instruments cannot access surfaces to characterize them directly. In other instances, a component or system must be dismantled or destroyed to measure a feature.
Over the years, many materials have been developed to replicate surfaces, enabling remote, nondestructive analysis. Early materials were effective for replicating relatively rough surfaces, but they had their limits. More recently, materials have been developed that can replicate features down to the nanometer scale. Used in conjunction with high-resolution measurement techniques, replication now enables monitoring of critical surfaces that are as large as aircraft wings or as small as inter-crystalline cracks.
The term "replication" often conjures an image of cellulose acetate, an early replicating material. Acetate softens and conforms to an underlying surface when it is wet with solvent. The process is somewhat involved, and the solvents may react with sample surfaces. For these reasons, acetate is now used primarily in labs for microscopic examination of metals. In its place, a variety of new materials have been developed to quickly replicate surface features.
Pressure-sensitive films produce fast and inexpensive replicas. In this method, the user rubs or burnishes a piece of foam-backed film to conform it to the sample surface. These films are used for rough surfaces such as grit-blasted metals in a surface roughness range (Ra) of about 20 to 120 microns. The films are also useful for measuring printing rollers, sheet products, and vertical or inverted surfaces.
Vinyl polysiloxanes, originally developed for dental impressions, have excellent dimensional stability, flow easily and have short curing times. The two-part materials are typically mixed and applied to the surface by hand. With resolution typically on the order of 1 micron, "dental washes" provide a mid-cost, mid-resolution replication solution.
Most recently, two-part silicone rubbers have been developed for high-resolution replication. These compounds, which are typically applied with a dispensing gun, can reproduce features to below 0.1 micron. Some can be used over a wide temperature range and can be applied underwater for corrosion, wear and damage inspection.
The variety of replication materials is a boon for metrology and quality experts-yet this wealth of options is not universally known. "People still think of acetate when they think of replication," notes Vic Rollins, managing director of Microset Products Ltd. (Nuneaton, UK), a manufacturer of high-resolution replication materials. "But we are replicating areas 6-feet long by 1-foot wide, with sub-micron resolution over the entire surface. This is the scale of things we can do today."
Analyzing material performance
After a replica has been made, various methods are available for measuring its surface. Optical profiling, or white-light interferometry, is particularly suited to measuring replications in lab or production environments. An optical profiler can measure feature heights from nanometers to millimeters, with sub-nanometer resolution, in only a few seconds. Because profiler data is three-dimensional, a user can monitor such parameters as directional wear, lay and fluid retention, as well as average roughness.
As a comparison of capabilities, a range of roughness and step-height samples were replicated using three representative materials: pressure-sensitive films, dental wash and two-part silicone rubber. All measurements were made using an optical profiler. The materials' ability to replicate lateral dimensions on several grating standards was also compared.
For measuring roughness, all three types of materials performed well over a range from 0.4 to 17 microns Ra. All three materials were able to accurately replicate lateral dimensions as well. Pressure-sensitive tape has been shown to perform well on grit-blasted surfaces up to at least 75 microns Rz (an averaged peak-to-valley parameter), but it did not perform as well on tall steps. According to Bob Stachnik, senior scientist at Testex (Monroeville, PA), a maker of pressure-sensitive films, there is some evidence that film's replication accuracy is a function of the character of the surface.
Which replication material to use depends on the application, cost, ease of use and the required resolution. Chris Pelow, research technologist for Alcan (Montreal), manufacturer of aluminum and packaging materials, uses pressure-sensitive film to monitor wear on mill rolls in a sheet aluminum operation. "We did a study to compare acetate to compressible foam films," says Pelow. "We now use the foam film because the process is fast and simple, it requires no chemicals and the technique is repeatable. This allows our operators to do their own measurements for later study."
Using 3-D analysis, Pelow monitors such parameters as average roughness, bearing ratio and peak spacing to understand the effects of various preparation techniques on the roll's performance and wear. "In addition to helping us with quality control, these measurements help us to engineer the roll surface. Fast, regular replication is key to that engineering."
Other applications require the high resolution and other advantages of two-part compounds. Microset's Rollins describes an application that requires roughness characterization of fuel injector ports. The 300-micron diameter ports are positioned at right angles to the part's single entry port. Using a two-part polymer, engineers were able to form a high-resolution cast that could be extracted in one piece from the part. Rollins cites the material's ability to stretch during extraction and then return to its after-curing dimensions as the reason such a measurement is possible.
Today's replication techniques and materials have matured to the degree that quality professionals can rely on replicated surfaces for high-resolution process monitoring and quality control. Improvements in resolution, ease of use and availability have made replication a viable solution for a wide range of precision measurements.
• Materials have been developed that can replicate features down to the nanometer scale.
• Used in conjunction with high-resolution measurement techniques, replication now enables monitoring of critical surfaces as large as aircraft wings or as small as inter-crystalline cracks.
• Most recently, two-part silicone rubbers have been developed for high-resolution replication.
• Optical profiling, or white light interferometry, is suited to measuring replications in lab or production environments.